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Amino acids are divided into essential (cannot be manufactured by the body and, therefore, must be taken in food) and non-essential. Amino acids are the building blocks of protein, including human protein.

Although cells synthesise many different chemicals, a large part of the cellular machinery is devoted to producing proteins. Proteins determine the physical and chemical characteristics of cells and so are vitally important. Protein is used to produce enzymes, some hormones, organelles, muscle (e.g. actin and myosin), structural components of skin and hair (e.g. collagen and keratin), plasma (blood) protein, antibodies, etc. The instructions for producing proteins are coded within the genes.

"Genes that work in the immune system code for proteins that do things like binding disease-causing parasites, presenting the bound parasite to executioner cells, and chemically dissolving the parasites. These activities correspond to directions both direct and implicit like 'detect the enemy', 'here's one of the bastards', and 'kill'." Page 31 Mendel's Demon Mark Ridley.

Proteins consist of amino acids that always contain carbon, hydrogen, oxygen and nitrogen. Proteins are giant molecules made by linking large numbers of amino acids, end to end, so they form a chain. In nature more than 100 amino acids are found, but only 20 are used in humans. The reason for this is that these 20 provide all the chemical and size groups needed to make a very large number of proteins. These 20 different amino acids join up in a variety of ways to make approximately 250,000 different human proteins. The same amino acid can occur many times along a chain making up a specific protein.